Semiconductor processing frequently involves providing a photoresist layer over a substrate. Portions of the photoresist layer are subsequently exposed to light through a masked light source. The mask contains clear and opaque features defining a pattern to be created in the photoresist layer. Regions of the photoresist layer which are exposed to light are made either soluble or insoluble in a solvent. If the exposed regions are soluble, a positive image of the mask is produced in the photoresist. The photoresist is therefore termed a positive photoresist. On the other hand, if the non-irradiated regions are dissolved by the solvent, a negative image results. Hence, the photoresist is referred to as a negative photoresist.
A difficulty that can occur when exposing photoresist to radiation is that waves of radiation can propagate through the photoresist to a layer beneath the photoresist and then be reflected back up through the photoresist to interact with other waves propagating through the photoresist. The reflected waves can constructively and/or destructively interfere with other waves propagating through the photoresist to create periodic variations of light intensity within the photoresist. Such variations of light intensity can cause the photoresist to receive non-uniform doses of energy throughout its thickness. The non-uniform dose can decrease the accuracy and precision with which a masked pattern is transferred to the photoresist. Also, the radiated waves reflected back from a non-flat surface underlying photoresist can enter portions of the photoresist that are not supposed to be exposed. Accordingly, it is desired to develop methods which suppress radiation waves from being reflected by layers beneath a photoresist layer.
A method which has been used with some success to suppress reflected waves is to form an antireflective material beneath a photoresist layer. Antireflective materials can, for example, comprise materials which absorb radiation, and which therefore quench reflection of the radiation.
Antireflective materials absorb various wavelengths of radiation with varying effectiveness. The wavelengths absorbed, and the effectiveness with which they are absorbed, vary depending on the materials utilized. The number of materials available for use as antireflective materials is limited. Accordingly, it is desired to develop alternative methods of varying the wavelengths absorbed, and effectiveness with which the wavelengths are absorbed, for antireflective materials.